Pressure-induced boron clathrates with ambient-pressure superconductivity

Abstract

Element B in binary metal borides forms various polymetric configurations due to its electron-deficient nature. Here we predict a new pressure-stabilized boron clathrate structure, LaB₈, that is recoverable under ambient conditions. Crystal structure searches and first-principles calculations predict a series of thermodynamically stable La–B compounds at high pressures. Besides known LaB₄ and LaB₆ compounds, trigonal LaB₈ is predicted to be thermodynamically stable above 70 GPa. Its B atoms construct face-sharing B₂₆ cages, each surrounding a central La atom. The compound can be recovered to ambient conditions with the B₂₆ cages well preserved. Electron–phonon coupling calculations suggest that it is potentially superconducting with an estimated T_c of 5 K at 70 GPa, which increases to ∼20 K at ambient pressure. The enhanced T_c is attributed to the increased B-derived electronic density of states at the Fermi level and to the softened E_u mode related to vibration of the B₂₆ cages.